Thin provisioning is a method used by storage systems to optimize utilization of available physical storage space. Instead of allocating all requested physical blocks (also known as storage regions) to data volumes (i.e., logical volumes) up front, thin provisioning implements on-demand allocation of the storage units to the data volumes. This methodology helps eliminate almost all “whitespace” (i.e., storage regions allocated to data volumes but not storing any data), thereby improving storage utilization rates over storage allocation methods such as thick provisioning that allocate, to individual data volumes, storage regions that may remain unused (i.e., not storing any data).
Thin provisioning implements an over-allocation (or over-subscription) mechanism that enables a storage system to view additional storage capacity than has been physically reserved on the storage system itself. In thin provisioning, physical storage space is either physical storage space or logical storage space, wherein the physical storage space is either reserved or used. Reserved physical storage space comprises an amount of storage space that is allocated to a storage pool, and used storage space comprises an amount of reserved physical storage space that is currently used to store data. For a given data volume, the logical space comprises the given volume's reported capacity, and for a given storage pool, the logical space comprises a maximum size for all data volumes in the pool.
Over-allocation enables flexibility in growth of data volumes, without having to predict accurately how much a given data volume will grow. Instead, storage region growth becomes sequential. Physical storage capacity in the storage system is only dedicated when data is actually written by a host computer, not when the storage volume is initially allocated. The servers, and by extension the applications that reside on them, view a full size data volume from the storage but the storage itself only allocates the blocks of data when they are written.
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